In the production of cement clinker the exhaust gases from the final burning zone (generally formed by a rotary kiln) usually contain a greater or lesser proportion of nitrogen oxides (so-called NO.sub.x content). These nitrogen oxides are formed partly by reaction of oxygen with the nitrogen in the combustion air and partly by oxidation of the nitrogen compounds in the fuel (cf. "Zement-Kalk-Gips" 37/1984, p. 499-507).
The NO.sub.x content of the exhaust gases is undesirable because of the emission problems which it causes. Therefore various methods have already been developed in order to reduce the NO.sub.x content of the exhaust gases.
Thus it is known in precalcination processes to produce a reducing atmosphere in the precalcination zone outside the rotary kiln by sub-stoichiometric combustion of fuel ("ICS proceedings" 1979, p. 45, FIG. 6). In this case the calcination of the preheated material is carried out in two stages: The first stage is operated with exhaust air from the cooling zone with an oxygen deficiency so that a CO-containing gas mixture from this precalcination zone enters the kiln exhaust gas duct. In this kiln exhaust gas duct a reducing atmosphere is first of all created in which a proportion of the NO.sub.x is reduced to nitrogen. In the adjoining part of the kiln exhaust gas duct combustion conditions are created by introducing a further proportion of exhaust air from the cooler so that the residual burn-out of the fuel takes place.
DE-A-35 22 883 also describes a process in which additional fuel is introduced into the exhaust gases from the rotary kiln outside the rotary kiln before the exhaust gases enter the precalcination zone, the quantity of this additional fuel and the oxygen content of the exhaust gases from the burning zone being such that a substoichiometric combustion in the exhaust gases from the burning zone takes place in the zone before the entry of the exhaust gases into the precalcination zone.
DE-A-39 25 475.5 also relates to a process in which a proportion of the fuel used for the entire heat treatment for the purpose of producing a reducing atmosphere and at least one fortifying reagent which promotes the effect of this reducing atmosphere on the NO.sub.x reduction are introduced into the material feed zone of the rotary kiln.
The exhaust gases from plants for the production of cement clinker frequently contain in addition to the NO.sub.x discussed above NH.sub.3, SO.sub.x, HCL as well as heavy metals and trace pollutants which can lead to emission problems. Until now there has been no method available which is both effective and economic for the removal of these pollutants.
The object of the invention, therefore, is to develop a method and a plant which make it possible using very effective and at the same time economic means to produce a marked reduction in the NO.sub.x content of the exhaust gases and at the same time to purify the exhaust gases of troublesome heavy metals and trace pollutants.